The procedures involved were not major "open" operations, but invovled the insertion of probes into blood vessels.

Despite the time it took to instruct patients in the techniques, the fact that patients under hypnosis were less restless meant that they actually spent less time - 17 minutes on average - in the operating theatre.

However, it is not clear whether the patient benefits found in the study were the result of actual hypnosis or simply deep relaxation triggered by the techniques.

Dr Peter Wallace, a consultant anaesthetist from the Western Infirmary in Glasgow, said that anything which reduced the need for drugs would be welcome.

He said: "The concept of relaxation techniques for patients undergoing this sort of procedure is well-established.

"I'm not sure whether there is any real difference between these and hypnosis, but certainly, the results here in terms of reduced need for sedatives is quite remarkable."

However, the president of the Royal College of Psychiatrists, Professor Leo Strunin, said that many hospital departments carrying out these procedures were too stretched to spare the time to teach such hypnosis techniques.

He said: "The problem is that they are so unpredictable - they may work for some people but not for others."